[jdk17] RFR: 8269230: C2: main loop in micro benchmark never executed [v2]

[Roland Westrelin]

> The benchmark loop is roughly:
> 
> for (int i = 0; i < limit; i++) {
>   if (!(i < max_jint && i > min_jint)) {
>     uncommon_trap();
>   }
> }
> 
> which IfNode::fold_compares() tranforms to:
> 
> for (int i = 0; i < limit; i++) {
>   if (!(i - min_jint - 1 <u -2)) {
>     uncommon_trap();
>   }
> }
> 
> Range elimination then adjusts loop limits so the unsigned comparison
> can be removed. The new limit is:
> 
> min(limit, max(-2 + min_jint + 1, min_jint)) = min(limit, min_jint) = min_jint
> 
> and the main loop is never executed.
> 
> The logic that protects against overflows during range check
> elimination is what causes the main loop to never be executed. As a
> fix, I propose delaying IfNode::fold_compares() if the resulting test
> compares with a potential negative number which could then cause RC
> overflow code to trigger. I think that would preserve the most
> interesting cases for IfNode::fold_compares() when it comes to range
> check elimination:
> 
> i >= 0 && i < array.length

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

  benchmark

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk17/pull/156/files
  - new: https://git.openjdk.java.net/jdk17/pull/156/files/21cc2e4e..f89c063a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk17&pr=156&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk17&pr=156&range=00-01

  Stats: 137 lines in 1 file changed: 137 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/156.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/156/head:pull/156

PR: https://git.openjdk.java.net/jdk17/pull/156